1. Field of the Invention
The present invention relates to a tire having a stiffener formed of a rubber strip, and a method for fabricating a bead member having a bead core and the stiffener.
2. Description of the Related Art
A tire includes a bead member provided at a bead portion, and a carcass ply. The bead member includes a bead core and a stiffener formed on the outer circumference of the bead core. The carcass ply is folded back around the bead core. The tire also includes a sidewall portion that is deflected and deformed during driving. Due to this deflection and deformation, an end of the carcass ply may be repeatedly subjected to large compression strain. If the end of the carcass ply is positioned on the stiffener, deformation and strain act on the stiffener. In order to counter this problem, Japanese Unexamined Patent Application Publication No. 11-11118 discloses a tire having a stiffener composed of two kinds of rubber to reduce deformation and strain.
In a conventional tire, a stiffener is composed of an inner rubber portion located on the radially inner side and an outer rubber portion located on the radially outer side. The inner rubber portion is a hard stiffener composed of rubber having a high rigidity. The outer rubber portion is a soft stiffener composed of rubber having a low rigidity. Because the hard inner rubber portion reduces deflection and deformation occurring at the sidewall portion, strain occurring at the end of the carcass ply is reduced. At the same time, the soft outer rubber portion absorbs and reduces strain that is being transmitted. Because the strain is reduced, the conventional tire can prevent separation occurring near the end of the carcass ply, even after a long-time drive.
Typically, the stiffener of a bead member is formed by wrapping wide extruded rubber around the bead core. The extruded rubber is formed in a predetermined shape by an unvulcanized rubber extruder. Extruded rubber having a high rigidity is wound around the bead core in one layer, forming the inner rubber portion of the stiffener. Extruded rubber having a low rigidity is wound around the inner rubber portion in one layer, forming the outer rubber portion of the stiffener. By using the extruded rubber, the time and effort required for forming the stiffener can be reduced, improving the production efficiency.
However, when the extruded rubber is used, because the ends thereof are joined, a joined portion is formed at one position in the circumferential direction. Furthermore, another joined portion is formed at the boundary between the inner and outer rubber portions. As a result, the surface of the stiffener becomes irregular. Thus, stiffeners made of extruded rubber needs to be improved, from the standpoint of further increasing the roundness and the balance in the circumferential direction. Furthermore, hardware (for example, the sleeve of the extruder) needs to be changed every time the shape of the stiffener is changed, which is time and cost consuming. To overcome this problem, Japanese Unexamined Patent Application Publication No. 2007-76233 discloses a method for forming a stiffener by laminating rubber strips (ribbon-shaped rubber members).
In this method, first, an unvulcanized rubber strip is laminated, forming a first rubber member. Then, a bead core having a hexagonal cross section is pressed against the first rubber member, integrating the bead core and the first rubber member. Next, a rubber strip is laminated on the bead core and the first rubber member, forming a second rubber member. The first and second rubber members are composed of the same rubber and together constitute the inner rubber portion of the stiffener. Then, a rubber strip is laminated thereon, forming a third rubber member. The third rubber member is composed of rubber softer than the first and second rubber members and constitutes the outer rubber portion of the stiffener. By forming the first to third rubber members in sequence, the stiffener is formed on the bead core.
By forming the stiffener of the rubber strips, the irregularity on the stiffener is reduced, and the roundness and the balance in the circumferential direction of the stiffener are improved. Furthermore, stiffeners of various shapes can be formed without changing hardware. However, stiffeners made of rubber strips are more susceptible to the above-described deformation and strain occurring at the end of the carcass ply than stiffeners made of extruded rubber. Therefore, it is difficult to improve the durability of the stiffeners made of rubber strips at the end of the carcass ply, and a further improvement in durability is required.